Computers, personal digital assistants, cellular telephones and other electronic systems and devices typically include processors and memory. The memory is used to store instructions (typically in the form of computer programs) to be executed and/or data to be operated on by the processors to achieve the functionality of the device. Some applications require retention of the instructions and/or data in a permanent or non-volatile storage medium. Such memories maintain information when the device is turned off or power is removed. Exemplary applications include computer Basic Input Output Systems (BIOS) storage and diskless handheld computing devices, such as personal digital assistants.
Flash memories store information in the absence of continuous power and are capable of being constructed in a very compact form. Flash memory is typically constructed by fabricating a plurality of floating gate transistors in a silicon substrate to form a core cell area. A floating gate transistor is capable of storing electrical charge on a separate gate electrode, known as a floating gate, that is separated by a dielectric layer from a control gate electrode. Generally, stored electrical charge in the floating gate represents a data state.
Flash memory devices also include a periphery area in the silicon substrate. Logic and addressing functions are performed by a plurality of logic and addressing circuits in the periphery area. The logic and addressing circuits perform internal logic operations such as reading, programming and erasing the stored charge in the floating gate transistors.
Flash memory devices also use a supply voltage (Vcc). The supply voltage (Vcc) is generated by a fixed voltage power supply that is used to power the internal logic operations.
The flash memory is electrically interfaced with a system processor of an electronic system or device. When the operation of the flash memory is desired by the electronic system or device, the flash memory is selected with a chip enable signal. When selected by the typically logic low (logic "0") chip enable signal, the flash memory's logic and addressing circuitry is activated such that the flash memory can be operated by the electronic system or device.
When the flash memory is deselected by the chip enable signal that is typically logic high (logic "1"), it is in a standby mode. The standby mode decreases the power consumption of the flash memory because less of the logic and addressing circuitry is activated. It is desirable to have a low standby current draw to maintain the lowest possible power consumption of the flash memory during standby mode.
Typically, the supply voltage (Vcc) originates from the same supply voltage that powers the electronic system or device. As such, the chip enable signal is generated by the electronic system or device using the supply voltage (Vcc). In some cases, the electronic system or device is powered from another power supply source that is used to generate the chip enable signal.
A problem occurs when the electronic system generates the chip enable signal at voltage levels that are lower than the supply voltage (Vcc). When the voltage level of the chip enable signal that is logic high is less than the supply voltage (Vcc), the logic circuit within the flash memory that receives the chip enable signal may not operate properly. Since the logic circuit is operated with the supply voltage (Vcc), the chip enable signal used to deselect the flash memory may not be capable of completely deactivating the logic circuit. As such, when the flash memory is deselected and enters the standby mode, leakage current may occur that increases the standby current and the flash memory will consume more power.